Circadian rhythmicity is a fundamental aspect of human physiology. Disruption of circadian rhythm is a significant health burden, impacting sleep, cardiovascular, metabolic and psychiatric disorders. Understanding how circadian modulation of the body's physiology contributes to human health and disease requires a basic understanding of how circadian rhythms are generated and expressed. The overall goal of the proposed research is to identify the basic mechanisms of that encode circadian rhythmicity in the suprachiasmatic nucleus (SCN), the brain's clock. The SCN circuit undergoes synchronized daily oscillations in action potential (AP) firing, and circadian behavioral and physiological characteristics are established by this SCN circuit rhythm. Daily modulation of ion channel activity is a critical basis for generating the circadian rhythm in neuronal activity in the SCN. Oe such ion channel, the BK calcium- and voltage-activated potassium channel (Kcnma1) is a central regulator of circadian rhythm. Daily modulation of BK current magnitude drives SCN circuit rhythmicity, and loss of BK channel function disrupts circadian behavioral and physiological rhythms. The day-night difference in BK current level is mediated by the ?2 subunit, which causes inactivation of BK channels during the day. The circadian variation in BK current properties based on ?2 function is unique, but it is not clear how ?2 regulates AP activiy to shape the rhythms in SCN neuronal activity. The proposed studies test the hypothesis that ?2-mediated inactivation is the critical property required for the BK channel's dynamic role in SCN AP firing, and that this process is central to circadian behavior. This hypothesis will be investigated using electrophysiological recordings of BK currents and APs in acute SCN brain slices, providing data to correlate changes in AP waveforms with inactivating properties of the ?2 subunit. Furthermore, the consequences for inactivation of BK currents that stem from circadian changes in BK's calcium source will be investigated. Lastly, the impact of a human epilepsy-linked mutation and other single nucleotide polymorphisms (SNPs) that vary BK/?2 current properties will be tested and the relevance to SCN and behavioral rhythmicity will be determined. The outcome of this project will be an understanding of how the daily regulation of BK currents governs SCN excitability, providing new physiological and translational insight into the mechanisms that influence human BK channel activity.